cracked frame due to seat post?

Wokie
Wokie Posts: 16
edited October 2010 in Workshop
My 6 month old alu frame has developed a crack around the top weld leading back, where the top tube meets the seat post tube. Its a good quality alu. I can only wonder that the cause, is the super strong Thomson seat post, which was just too strong, with no give at all. I had the seat post set high(well under max), with plenty of seat post left in the frame.

Possibly If i had used a carbon or more flexible, less strong post, it might not of happened. What you think?

Comments

  • Robert456
    Robert456 Posts: 103
    If you've used the post correctly hopefully this is a warranty issue, might just be a one off dodgy frame
  • crankycrank
    crankycrank Posts: 1,830
    Pictures?? Probably more of a manufacturing defect. Type of seat post should make no difference, each will be supporting the same amount of weight :wink: .
  • rake
    rake Posts: 3,204
    how much post do you have inside the frame? a longer post has many benefits :wink:
  • waterford123
    waterford123 Posts: 172
    Faulty frame i'd counter.
  • pete.whelan
    pete.whelan Posts: 788
    if the length of seat tube about the weld is only a1-2cm then it is quite likely to crack due to the welding stress
    Recipe: shave legs sparingly, rub in embrocation and drizzle with freshly squeezed baby oil.
  • Monty Dog
    Monty Dog Posts: 20,614
    Excessively exposed seatpost i.e. long leverage / lots of setback and a heavier rider all conspire to put a lot of stress on the seatube weld and can contribute to problems - cracks generally propagate from an existing fault/flaw/stress raiser - unlikely that you'd suffer a fatigue failure within such a relatively short period.
    Make mine an Italian, with Campagnolo on the side..
  • rake
    rake Posts: 3,204
    Monty Dog wrote:
    Excessively exposed seatpost i.e. long leverage / lots of setback and a heavier rider all conspire to put a lot of stress on the seatube weld and can contribute to problems - cracks generally propagate from an existing fault/flaw/stress raiser - unlikely that you'd suffer a fatigue failure within such a relatively short period.
    thats what i meant. im quite heavy and have a lot of exposed seatpost, so i specifically got a long post so i had a good amount inside the frame which will reduce the tube busting leverage and spread it further down away from the tube joints. the weight isnt really much extra on an al post. minimum insertion marks are on the optimistic side i think.
  • Wokie
    Wokie Posts: 16
    edited October 2010
    Can't say how much, a 350mm length with plenty still in the tube. It probably really is a duff frame weld. I got warranty and changed frame.

    Most frames have 2cm seat frame tube protruding above the top tube.

    I don't think i would of suffered this problem on a carbon frame.
  • mozami
    mozami Posts: 25
    The frame wasnt a gary fisher 29er by any chance was it?
    Sometimes the minimum insertion mark on the seat post should be ignored if the seat tube continues a fair bit above the seat tube/top tube junction.

    You should make sure there is enough seat post below the point the top tube meets the seat tube, which can be quite low on sloping frame designs. If you use so much exposed seatpost why didnt you just get a bigger size frame in the first place?

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